In recent years, with respect to withstand load performance required in a gear oil for automobiles, levels of from GL-4 to GL-5 in terms of gear oil designation by the API (American Petroleum Institute) are being required, as a result of increased automobile power.
In gear oils required to have such durability, for the purpose of maintaining an oil film on the gear tooth flanks, it was common to employ oil of viscosity No. 90 (13.5 to 24.0 mm2/s (100° C.)) as defined by the SAE (Society of Automotive Engineers).
However, on the other hand, fuel consumption saving is also required; for the purpose of realizing this, the resistance to stirring must be reduced; and in order to achieve this, it is required to make the viscosity low.
In order to meet both of these requirements, namely, of maintaining the action of forming an oil film on the gear tooth flanks and of making the viscosity low, if the conventional technique is adopted of increasing the amount of extreme pressure additives added to a base oil of low viscosity, there is a high risk that phosphorus or sulfur-based additives used as the extreme pressure additive may have the adverse effect of increasing corrosion of parts containing a copper constituent, thereby shortening the life of the equipment. Accordingly, an additive composition for gear oil capable of reducing the corrosion of such copper or copper alloys has also been proposed. See JP-A No. 2004-323850.